1. Field of the Invention
This invention relates to a positive air pressure attenuation device for relieving positive pressure transients in drainage or sewage systems.
2. Description of the Prior Art
In such drainage systems, and in particular in buildings having more than three levels, the use of air admittance valves are required to prevent the release of foul air, and to admit air under conditions of reduced pressure (negative pressure) in the discharge pipes and stacks. Such valves are disclosed f.i. in patent documents GB 2.014.697, EP 0 084 303, EP 0 278 746 or EP 1 026 329.
However, the use of these valves prevent positive air pressure transients, arising within the drainage system, from escaping to the atmosphere, leading to a reduced performance of the drainage system.
Patent documents EP 0 172 966 and EP 0 661 483 disclose sewer relief valves which automatically open responsive to overpressure due to a back flow of sewage and drain water within a plumbed drainage system.
According to these embodiments however, the overpressure is relieved or attenuated by expelling air from the sewage system into the atmospheric air.
Such positive air pressure transients should be temporary attenuated within the system without expelling foul air from the system into the surrounding atmosphere.